1. Technical Field
The technical field relates to a lens barrel with which the focal length can be changed.
2. Description of the Related Art
Conventional digital cameras make use of a zoom lens system with which the focal length can be varied while the object distance of a subject that is in focus (hereinafter also referred to as the subject distance) is kept substantially constant. For example, zoom lens systems are employed in compact digital cameras and digital cameras with interchangeable lenses.
With a conventional lens barrel, for example, as the zoom mechanism operates, the focus lens unit including the focus lens is moved in the optical axis direction by a cam mechanism. This allows the focal length to be varied while the subject distance is kept substantially constant (see, for example, Japanese Laid-Open Patent Application 2006-113289).
A phase difference detection system has been employed as the auto-focus system with conventional interchangeable lens digital cameras.
More recently, however, an interchangeable lens digital camera has been proposed that makes use of a contrast detection system for auto-focusing. With this contrast detection system, for example, the focus lens unit is moved in the optical axis direction while evaluation values at various positions of the focus lens unit are found on the basis of image data. The focus lens unit is moved until the evaluation value goes past its peak, after which the evaluation value is returned to its maximum position to focus the subject image (an optical image of the subject). Thus, in auto-focusing by contrast detection, it is necessary to move the focus lens unit back and forth in the optical axis direction.
Also, since the focus needs to be continued during the capture of moving pictures, the focus lens unit has to be continuously moved back and forth and the peak of the evaluation value detected.
Thus, when a contrast detection system is used, since the focus lens unit is moved in the optical axis direction, making the focus lens unit smaller is preferable when drive speed is taken into account.
However, when a configuration is employed in which the focus lens unit is driven in the optical axis direction by a cam mechanism, as with the lens barrel described in Japanese Laid-Open Patent Application 2006-113289, the focus lens unit ends up being larger or heavier.
In view of this, the inventors of the present application studied a lens barrel with which drive of the zoom mechanism was only performed during manual operation by the user, and drive of the focus lens unit with respect to the zoom mechanism was performed only by an actuator. In this case, because the structure of the focus lens unit and its surrounding components is simplified, the focus lens unit can be smaller.
However, with this lens barrel, since the focus lens is driven by an actuator, if the user operates the zoom mechanism in a state in which no power is supplied to the actuator, there is the possibility of a significant discrepancy in the relationship between the zoom position of the optical system and the position where the focus lens unit is supposed to be. For example, if the focus lens unit needs to be moved to a specific position corresponding to the zoom position, depending on the zoom position of the optical system at the point when the power is turned on, it may take a long time for the focus lens unit to move to the specified position.
Japanese Laid-Open Patent Application 07-199033 discloses a resetting unit for resetting the position of a lens support frame to a specific reference position on the basis of output from a step-out detection unit for detecting step-out of the lens support frame from the actuator, but a constitution that involves reducing start-up time has yet to be proposed.